1. Field of the Invention
The invention relates to a sheet transporting apparatus, and particularly to the correction of the position of a sheet in the cross direction thereof perpendicular to a sheet transport direction and the correction of the skew feed posture of the sheet relative to the sheet transport direction.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, or an image reading apparatus, a registration (skew feed correction) device for the posture correction and alignment of a sheet being transported is provided immediately before the image forming portion or the image reading portion thereof. As such a registration device, there is, for example, an active registration system for correcting the skew feed of a sheet while transporting the sheet. This system is such that two original detecting sensors are disposed in a sheet transport path in the cross direction (hereinafter referred to as the main scan direction) of the sheet perpendicular to a sheet transport direction, and detect the skew of the leading edge of the sheet on the basis of a signal produced by the leading edge of the sheet crossing the respective sensors, and also the sheet transporting speed of skew feed correction rollers (hereinafter referred to as the registration rollers) disposed in the main scan direction and drivable independently of each other is controlled to thereby correct the skew feed of the sheet.
According to this system, skew feed correction can be effected while the sheet is transported without being once stopped and therefore, the throughput of the sheet is not reduced. FIG. 13 of the accompanying drawings is a typical view illustrating a method of correcting the skew feed of a sheet transported by the active registration system.
As the above-described conventional skew feed correction control method, there is an acceleration and deceleration control method or the like as shown in FIG. 13 wherein a transport roller 14 on one side preceding by skew feed is deceleratedly driven (the arrow D in FIG. 13) and at the same time, the accelerated drive of a transporting roller 13 (the arrow A in FIG. 13) on the other side delayed is effected.
On the other hand, the skew feed direction, the detected skew feed amount (Nb), etc. of an original transported by rollers 13 and 14 are measured by original detecting sensors 15 and 16. As drive sources for rotatively driving the rollers 13 and 14, use is usually made of pulse motors 11 and 12, and the aforementioned detected skew feed amount Nb is measured by counting the time from after one of the original detecting sensors 15 and 16 has detected the original until the other sensor detects the original, by counting clocks driving the pulse motors 11 and 12.
Further, the original detecting sensors 15 and 16 are disposed at distances equal from the center of the main scan direction to the right and left and therefore, there is the characteristic that when skew feed control is effected in conformity with the detected skew feed amount Nb calculated from the detection information of these detecting sensors 15 and 16, the transport of the original in the central portion of the main scan direction of the original does not differ from the ordinary transport thereof, in the case of the acceleration and deceleration control method.
However, an improvement in the sheet transporting speed has sometimes caused, in addition to the skew feed of the sheet, the deviation of the sheet position in the main scan direction.
By the conventional active registration system, it has been impossible to detect the deviation of the sheet position in the main scan direction and therefore, to correct the deviation of the sheet position.